Uranium dioxide for the manufacturing of current light water reactor fuel is currently produced from the conversion of UF.sub.6, mainly based either on a dry- or a wet-conversion process. Several routes of the dry-conversion process have been revealed so far, and chemical procedures involved in those routes are similar. UF.sub.6 is usually pyrohydrolyzed with steam to form UO.sub.2 F.sub.2 powder which is reduced to UO.sub.2 directly by a hydrogen-steam mixture, or is calcined in air to U.sub.3 O.sub.8 first and then reduced to UO.sub.2 with a hydrogen-steam gas. In the wet-conversion process, vaporized UF.sub.6 is hydrolyzed with water to form an aqueous UO.sub.2 F.sub.2 -HF solution, from which ammonium diuranate (ADU) or ammonium uranyl carbonate (AUC) is precipitated with ammonia water or ammonium carbonate, respectively. After filtration, ADU or AUC is calcined to UO.sub.3, which is then reduced to UO.sub.2 with a hydrogen-steam gas. According to the chemical compositions of the precipitates, it is called an ADU process or an AUC process.
It is recognized that the UO.sub.2 powder produced from the wet-ADU process possesses excellent powder characteristics required for pelletizing and sintering, and gives good microstructure to the sintered pellet. Although the ADU process is widely used currently, it is plagued by some inherent drawbacks. For example, in the conventional ADU process, such as that disclosed in the U.S. Pat. Nos. 3,394,997 and 3,998,925, UF.sub.6 is hydrolyzed with water to form an aqueous solution containing 100 to 200 g/l of uranium and 0.4 to 0.8 mol/l of hydrogen fluoride. As ADU is precipitated from this solution, a pasty slurry is obtained and several tens of liters of the fluoride-containing liquid filtrate is thus generated for the production of 1 kg UO.sub.2. This introduces a serious problem of liquid waste disposal to the conventional ADU process. Moreover, because ADU is a kind of slimy cake, the process also involves a complicated filtration operation.
After a series of studies on the formation of ADU, it was found that ADU is formed simultaneously as soon as fine droplets of a concentrated solution of an uranyl compound are introduced into an ammonia gas stream, and the fluorine content of the UO.sub.2 powder consequently produced using uranyl fluoride solution as a feed can be lower than 50 ppm. Therefore, instead of being precipitated from a dilute solution of uranyl compounds with ammonia water, ADU is prepared in particle form directly by introducing atomized droplets of a concentrated solution of uranyl compound into an ammonia gas stream in the novel process disclosed herein. The generation of the fluoride-containing liquid filtrate in converting uranyl fluoride to UO.sub.2 is thus avoided, and filtration operation is no longer necessary. The process is thus greatly simplified.